This only solves the problem with SBCL code, which only uses (setf ([f]documentation 'structure-accessor 'function) "...")

But since structures share the same closure for accessors, setting documentation of #'structure-accessor will mess it up. It's not used this way in the sources of SBCL, but external libraries do that (cxml, for example). So it's sufficient just to load a library with a single (setf documentation) on a structure accessor to mess up all other accessor docstrings throughout SBCL.

Note that (setf fdocumentation) is susceptible to this problem when called on symbols. Although it can be easily fixed in the same manner as (setf documentation), I see no point in doing so, since it will be messed up by external libraries anyway.